Storing and retrieving data from a hard disk drive or other storage media is essential to modern electronics. A hard disk drive typically stores digitally encoded data on rapidly rotating platters having magnetic surfaces. Platters in a hard disk drive are usually rotated by a motor commonly referred to as a spindle motor.
One factor affecting the reliability of a hard disk drive is accurate control over the speed of rotation of platters (or the speed of a spindle motor). The speed of a spindle motor is typically controlled by a motor driver that provides power to the spindle motor using pulse-width modulation (PWM) techniques. As a result, an output of the motor driver (e.g., a pulse width modulated signal) switches between potentials of a positive power supply and a negative power supply (e.g., ground). This switching can cause electromagnetic interference (EMI) and noise that disrupts other sensitive circuits in the hard disk drive.
To reduce noise and EMI, the slew rate of the output of the motor driver needs to be controlled. However, controlling the slew rate with an inductor load present (such as an inductor load in a spindle motor) is difficult, especially during inductive recirculation when the output voltage quickly approaches the positive supply rail voltage. In addition, any slew rate controller needs to respond quickly to an input control signal to maintain the spindle motor at a constant speed, adding further difficulty to slew rate control.